This invention relates to a fluorescent luminous device, and more particularly to a fluorescent luminous device including a phosphor which prevents such a deterioration in cathode as caused by a sulfide phosphor and exhibits an increased emission spectrum and improved luminance characteristics.
In the art, there have been conventionally known a field emission display (FED) and a vacuum fluorescent display (VFD) as a fluorescent luminous device. A fluorescent luminous device for monochromatic display generally has a ZnO:Zn phosphor incorporated therein. The ZnO:Zn phosphor exhibits highly increased luminescence under a low voltage while ensuring increased reliability, however, it fails to emit light of a red luminous color because a luminous color thereof is bluish green. Multi-coloration of display in a conventional fluorescent luminous device is carried out by either using filters of various colors or coating an anode with various phosphors different in luminous color.
Techniques of arranging a filter on the ZnO:Zn phosphor to provide luminescence of various luminous colors cause a reduction in luminance of a reddish luminous color, to thereby fail to put the fluorescent luminous device to practical use. In view of such a disadvantage, it is proposed to add a phosphor of a reddish orange luminous color such as ZnCdS:Ag,Cl, Y.sub.2 O.sub.2 S:Eu or the like to the ZnO:Zn phosphor to permit the ZnO:Zn phosphor to exhibit a reddish luminous color as well, resulting in a reddish luminous color close to white which has a wide spectrum range being provided. Also, additional techniques of employing a phosphor of a white luminous color used in a cathode ray tube (CRT) are proposed.
Unfortunately, a phosphor obtained by adding such an additional phosphor as described above to the ZnO:Zn phosphor is varied in voltage dependency of luminance depending on a type of the phosphor added, leading to color shift wherein a luminous color is varied due to a variation in drive voltage. Also, the phosphor added to the ZnO:Zn phosphor is deteriorated in life characteristics as compared with the ZnO:Zn phosphor, leading to shift of chromaticity during lighting operation of the fluorescent luminous device. Further, the ZnCdS:Ag phosphor exhibits satisfactory luminance characteristics, however, it causes scattering of a sulfur ingredient during lighting operation of the device, to thereby deteriorate cathodes. Moreover, the phosphor for the CRT is inherently decreased in luminance and deteriorated in life characteristics.
Currently, in order to obtain luminous colors different from that of the ZnO:Zn phosphor by means of only one kind of phosphor, a sulfide phosphor is put to practical use. However, the sulfide phosphor causes decomposition and scattering thereof due to impingement of electrons thereon, leading to pollution of cathodes and therefore a deterioration thereof.
Scattering of a sulfur ingredient contained in the sulfur phosphor deteriorates both emission characteristics and life characteristics of filamentary cathodes. This affects field emission cathodes of an FED more seriously than the filamentary cathodes.
More specifically, an FED includes a cathode substrate formed on an inner surface thereof with field emission cathodes. The field emission cathodes each include a cathode conductor formed on the cathode substrate, an insulating layer formed on the cathode conductor, a gate electrode formed on the insulating layer, holes each formed in the gate electrode and insulating layer so as to commonly extend through the gate electrode and insulating layer, and emitters of a conical shape each arranged in each of the holes while being positioned on the cathode conductor. The FED also includes a light-permeable anode substrate arranged opposite to the field emission cathodes on the cathode substrate while being spaced from the cathode substrate at a micro-distance. The anode substrate is formed on an inner surface thereof with anodes. The anodes each include a light-permeable anode conductor and a phosphor layer deposited on the anode conductor. The cathode substrate and anode substrate are sealedly joined at an outer periphery thereof to each other by means of a seal material. In the FED thus constructed, application of a voltage of a suitable level to the cathode conductor and gate electrode permits electrons to be emitted from a distal end of the emitters, which are then impinged on the anode, leading to luminescence of the phosphor layer on the anode. Luminescence of the anode is then observed through the light-permeable anode conductor and anode substrate.
Field emission of the FED is based on an electron field-emitting phenomenon, resulting in being affected by a variation in work function due to adsorption of gas and fine particles on the field emission cathode. For example, adhesion of oxygen and sulfide gas to the distal end of the conical emitter increases a work function because an interval between the distal end of the emitter and the gate is microfine. Also, the field emission cathode and anode each are formed into a flat shape of a predetermined area and an interval therebetween is set to be as fine as, for example, 200 .mu.m, to thereby be highly susceptible to gas emitted from the anode. Further, gas emitted from the anode and sulfide gas adhered to the field emission cathode may possibly cause short-circuiting between the anode and the field emission cathode, leading to breakage of the anode and field emission cathode, resulting in emission characteristics being fully deteriorated. Thus, it will be noted that the field emission cathode is highly adversely affected by decomposition and scattering of the sulfide phosphor as compared with the filamentary cathode.
In view of the foregoing, the assignee proposed such a phosphor layer formed by lamination as shown in FIG. 10. This is disclosed in Japanese Patent Application Laid-Open Publication No. 55592/1996 (Japanese Patent Application No. 190590/1994). More particularly, the phosphor layer is formed by laminating a ZnO:Zn phosphor layer element 103 on a sulfide phosphor layer element 102 deposited on an anode conductor 101 formed on an anode substrate 100. Unfortunately, such an arrangement as shown in FIG. 10 causes the sulfide phosphor to be decomposed by a slight amount of electrons reaching the sulfide phosphor 102 through particles of the ZnO:Zn phosphor 103 as indicated at arrows in FIG. 10, leading to scattering of the sulfide phosphor. Thus, the arrangement fails to provide complete solution to the problem.
As described above, the techniques intended to obtain a desired luminous wavelength by mixing of the ZnO:Zn phosphor with any additional phosphor which were proposed in view of the fact that the ZnO:Zn phosphor lacks a luminous component of a red color fail to exhibit desired reliability due to a difference in life and luminance characteristics between the phosphors. Also, the sulfide phosphor currently used causes a deterioration in cathode. In particular, the field emission cathode (FEC) is critically damaged due to pollution by decomposition of the sulfide phosphor. Further, a phosphor increased in resistance is not suitable for use in the fluorescent luminous device because it causes anode voltage drop. Under such situations, it would be highly desirable to provide a fluorescent luminous device including a non-sulfide phosphor which is reduced in resistance and emits light of a reddish luminous color.